I had issues with one version of Sumutra (can't remember which because I deleted it).
But SumatraPDF-3.3.3-64.exe and the prelease work fine.
As a comparison, they load Pickwick Papers (953 pages) (see
https://www.gutenberg.org/ebooks/580.epub3.images ) in less than a second for version 3.3.3 . This is on my 5 year old Windows 10 machine.
Calibre Viewer on the other hand has always been slow and takes about 10 seconds before anything appears to happen and another 5 seconds until I can start reading.